Except for on-axis measurements, dimensions and locations of eye components behind the iris cannot be fully determined by optical means. Ultrasonic imaging in the frequency range of about 5 MHz to about 80 MHz can be applied to make accurate and precise measurements of structures of the eye, such as, for example, the cornea and lens.
An ultrasound scanning apparatus is described in the following patent applications, all of which are incorporated by reference:    1. U.S. patent application Ser. No. 12/347,674 entitled “Components for an Ultrasonic Arc Scanning Apparatus” filed Dec. 31, 2008;    2. U.S. patent application Ser. No. 12/418,392 entitled “Procedures for an Ultrasonic Arc Scanning Apparatus” filed Apr. 3, 2009;    3. U.S. patent application Ser. No. 12/475,322 entitled “Compound Scanning Head for an Ultrasonic Scanning Apparatus” filed May 29, 2009;    4. U.S. patent application Ser. No. 12/638,661 entitled “Alignment and Imaging of an Eye with an Ultrasonic Scanner” filed Dec. 15, 2009;    5. U.S. patent application Ser. No. 12/754,444 entitled “Method of Positioning a Patient for Medical Procedures” filed Apr. 5, 2010; and    6. U.S. patent application Ser. No. 13/684,699 entitled “Alignment and Imaging of an Eye with an Ultrasonic Scanner” filed Nov. 26, 2012.
Ultrasonic imaging has been used in corneal procedures such as LASIK to make accurate and precise images and maps of cornea thickness which include epithelial thickness, Bowman's layer and images of LASIK flaps. These images have an A-scan resolution of about 5 microns.
New procedures such as implantation of accommodative lenses may provide nearly perfect vision without spectacles or contact lenses. Implantation of accommodative lenses requires precision measurements of, for example, the position and width of the natural lens for successful lens powering and implantation. Ultrasonic imaging can be used to provide the required accurate images of the natural lens especially where the zonules attach the lens to the ciliary body which is well off-axis and behind the iris and therefore not accessible to optical imaging. Other new procedures such as implantation of stents in or near the suprachoroid may provide part or all of a treatment for glaucoma. Ultrasonic imaging can be used to provide the required accurate images in the corner of the eye between the sclera and the iris (in the suprachoroidal space) which is well off-axis and relatively inaccessible to optical imaging.
Such measurements provide ophthalmic researchers with valuable information that can be used 1) in the design of accommodative lenses, 2) provide ophthalmic surgeons with valuable information that can be used to guide various surgical procedures performed on the lens, 3) in the design of glaucoma stents, 4) provide ophthalmic surgeons with valuable information that can be used to guide placement of stents for treatment of glaucoma.
Recent advances in ultrasonic imaging have allowed images of substantially the entire lens capsule to be made. This has opened up the ability of diagnostic devices to assist in both research of lens implantation devices and strategies, and to planning, executing and follow-up diagnostics for corrective lens surgery including specialty procedures such as glaucoma and cataract treatments as well as implantation of clear intraocular lenses including accommodative lens.
The use of ultrasonic imaging of important features of the eye for lens implantation is discussed, for example, in U.S. Pat. No. 7,048,690. This patent does not include techniques for imaging the posterior surface of the lens capsule and so cannot be used to compute the volume of a lens capsule. Means for obtaining a full image of the lens capsule are disclosed in U.S. patent application Ser. No. 12/475,322 and U.S. patent application Ser. No. 12/638,661.
An ultrasonic scan of the eye may include one or more rapid B-scans (each B-scan formed from a plurality of A-scans) at each of several meridians (typically about 3 to about 12 meridians) and these may be combined automatically to form a comprehensive image of the anterior segment. Therefore it is necessary to rapidly scan a patient to reduce the possibility of patient eye motion during a scan session. Further, it may be necessary to re-scan a patient at a later time in order to determine if changes in features or dimensions has occurred.
The speed of transducer motion in an precision scanning device such as described, for example, in U.S. patent application Ser. No. 12/638,661, is limited because its movement is in a bath of water and excessive speed of motion of the transducer and its carriage can result in vibration of the entire instrument. In practice, a set of ultrasound scans can be carried out in about 1 to about 3 minutes from the time the patient's eye is immersed in water to the time the water is drained from the eyepiece. The actual scanning process itself can be carried out in several tens of seconds, after the operator or automated software completes the process of centration (centration means aligning the center of curvature of the scanning transducer in space with the center of curvature of the eye component of interest such that rays from the transducer pass substantially through both centers of curvature). As is often the case, the patient may move his or her head slightly or may move his or her eye in its socket during this time. In some cases, the patient's heart beat can be detected as a slight blurring of the images. If patient movements are large, the scan set can always be repeated.
It is also important to compensate for unintended patient head or eye motion because a scan of the anterior segment scan or lens capsule scan is typically made by overlaying two or three separate scans (such as an arcuate scan followed by two linear scans, also described in U.S. patent application Ser. No. 12/638,661.
There remains, therefore, a need for methods that can be used track unintended movements of the eye during scanning to provide a reliable reference for multiple scans in a scanning session. Additionally, these methods are required to track a reference point in an eye during scanning and to locate this reference point for scanning sessions conducted at a later time.